Elsevier

Fuel

Volume 87, Issue 6, May 2008, Pages 844-856
Fuel

Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties

https://doi.org/10.1016/j.fuel.2007.05.041Get rights and content

Abstract

Torrefaction is a treatment which serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation; for example, combustion, co-combustion with coal or gasification. The topic has gathered interest in the past two decades but further understanding is required for optimisation of the process thus enhancing economic efficiency, which is crucial to the success of the treatment commercially and within industry. In particular there is a noticeable gap in current literature regarding the combustion properties of torrefied biomass. This study examines torrefaction in nitrogen of two energy crops, reed canary grass and short rotation willow coppice (SRC), and a residue, wheat straw. Product evolution and mass and energy losses during torrefaction are measured using a range of laboratory scale methods. Experiments at different torrefaction conditions were undertaken to examine optimization of the process for the three fuels. Progress of torrefaction was also followed by chemical analysis (C, H, N, O, ash), and it was seen that the characters of the biomass fuels begin to resemble those of low rank coals in terms of the van Krevelen coal rank parameter. In addition, the results indicate that the volatile component of biomass is both reduced and altered producing a more thermally stable product, but also one that produces greater heats of reaction during combustion. The difference between the mass and energy yield was shown to improve for the higher torrefaction temperatures investigated. The combustion behaviour of raw and torrefied fuels was studied further by differential thermal analysis (DTA) and also, for willow, by suspending individual particles in a methane–air flame and following the progress of combustion by high-speed video. It is shown that both volatile and char combustion of the torrefied sample become more exothermic compared to the raw fuels, and that depending on the severity of the torrefaction conditions, the torrefied fuel can contain up to 96% of the original energy content on a mass basis. Upon exposure to a methane-air flame, torrefied willow ignites more quickly, presumably because its low moisture content means that it heats faster. Torrefied particles also begin char combustion quicker than the raw SRC particles, although char combustion is slower for the torrefied fuel.

Introduction

Biomass is a unique fuel and has the potential to play a significant role in the future energy mix in the UK. Unlike other renewables, biomass can provide continuous electricity generation, and is the only widespread source of renewable heat. Increased use of biomass as a source of energy (electricity and heat) will contribute to the reduction of CO2 emissions, increase energy security, and support sustainable development and regeneration of rural areas, both through increased agricultural and forestry activity and the provision of small scale localised energy and heat generation schemes [1].

However, renewables still only account for a minute proportion of energy generation in the UK. In 2005, 4.2% of electricity was generated by renewable sources, 53% of which was from biomass and wastes (including MSW (municipal solid waste), animal waste, forestry residues, industrial and domestic wood, co-fired generation and short rotation coppice) [2]. However, in 2004 renewable heat generated from biomass and biofuels accounted for just 1% of the total heat market [1]. Evidently there are a number of obstacles to overcome in order to expand the biomass sector and the use of biofuels. Whilst some of these include bureaucracy, economics and fragmented approaches from regional and national authorities, there are also limitations associated with biomass fuel properties.

Direct comparisons of biomass with coal, still the dominant solid fuel in electricity and heat generation, often reveal inferior properties of biomass. In particular, it has lower energy densities, is a bulkier fuel (with poorer handling and transportation characteristics), more tenacious (its fibrous nature means it is difficult to reduce to small homogeneous particles) and – in most cases – a higher moisture content, resulting in storage complications such as degradation and self heating. These properties can have negative impacts during energy conversion such as lower combustion efficiencies and gasifier design limitations.

Torrefaction is a thermal treatment that occurs in an inert atmosphere. It removes moisture and low weight organic volatile components and depolymerises the long polysaccharide chains, producing a hydrophobic solid product with an increased energy density (on a mass basis) and greatly increased grindability. As a result, significantly lower energy is require to process the torrefied fuel and it no longer requires separate handling facilities when co-fired with coal in existing power stations. (In 2005, 15% of all renewable electricity generation in the UK was from biomass co-fired with coal [2].) It has also been suggested that the modified fuel can be compacted into high grade pellets with substantially superior properties when compared with standard wood pellets. The process can be incorporated into a combined drying, torrefaction and pelletisation process, with both economic and energy efficiency benefits [3]. Finally, it has been suggested that torrefied biomass is a suitable feedstock for entrained flow gasification, systems previously not considered feasible for raw biomass solid fuels. This is because torrefied biomass forms more spherical shaped particles during grinding or milling [4]. However, the process requires a separate plant, an input of process energy and the production of gaseous and volatile streams, entailing capital costs, operating costs and emission control. The balance between these associated costs and energy consumption and the cost and energy benefits from a more grindable, higher calorific value fuel are therefore critical for the future of torrefaction and require thorough analysis and reliable, extensive data. A evaluation of the torrefaction of wood chips published in 2001 concluded that the environmental and heating value benefits gained by torrefaction are not significantly greater than the extra energy consumption and capital investment [5], although it did indicate potential for the torrefaction concept in fuel densification. However, a number of other publications are optimistic about the benefits torrefaction has to offer. Furthermore, there are concepts such as retrofitting existing power stations in order to ultilise waste heat to serve a torrefaction process plant which must be considered.

Torrefaction has only received significant interest in the last two decades, and has yet to become a commercial process. The majority of research to-date has focused on the compositional changes that occur in biomass, determined by proximate and ultimate analysis, along with mass and energy yields [6], [7], [8]. Some studies have also investigated the composition of volatile matter released during the process [9], [10]. This previous work has focused primarily on woody biomass, including the comparisons of softwoods and hardwoods. Energy crops have received little attention, and there is still a need for comprehensive understanding of the optimum torrefaction conditions, as well as investigations of the combustion behaviour of these fuels, including the emissions and smoke compositions from torrefied fuels. Work presented in this paper reports on fundamental investigations into torrefaction of an energy grass, and makes comparisons with a woody biomass (willow) and an agricultural bi-product (wheat straw). In the second half of the paper combustion properties of all three fuels are investigated. The results from this paper will contribute to further work involving economic analysis of this pre-treatment, which is beyond the scope of this paper.

Section snippets

Samples

Two energy crops and an agricultural residue, namely willow (short rotation coppice, SRC), reed canary grass and wheat straw respectively, have been torrefied with subsequent analysis of the solid residues. These types of lignocellulose biomass material cover two of the three solid biofuel categories currently being considered as suitable for thermochemical conversion processes in the UK. Standard DD CEN/TS 14961:2005 [11] categorises willow (short rotation coppice) as woody biomass (forest and

TGA-FTIR torrefaction

Fig. 1 illustrates the influence of temperature and residence time on the final mass yield of the solid residue. At 503 K there was only slight reduction in mass and less than the amount of moisture lost during drying. However, increasing temperature has a marked affect on the thermal decomposition of the biomass; for temperatures of 563 K mass losses of 27–38% were observed for the three samples on a dry basis. The greatest rate of mass loss occurred during the temperature ramping stage but the

Conclusions

Herbaceous biomass undergoes greater alterations in its properties during torrefaction than woody biomass, in terms of mass loss and atomic composition. The overall mass loss in both herbaceous species investigated was similar but wheat straw underwent greater changes in terms of it elemental composition and increases in energy content: carbon content rose by a maximum of 14% to 51 wt%, oxygen content was reduced by 30% to 25% and the energy content rose by 20% to 22.6 MJ kg−1. However, the

Acknowledgements

This work was funded by the EPSRC and carried out in the Supergen Consortium in Biomass, Biofuels and Energy Crops (GR/S28204). The authors wish to thank Rural Generation in Northern Ireland for providing the SRC, the Institute of Grassland and Environmental Research (IGER) for conducting dry matter digestibility (DMD) analysis, and EPSRC for the loan of the Photo-Sonics Phantom V7 high-speed video system. TGB thanks EPSRC for Doctoral Training support.

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